The present invention relates to the electroplating of chromium or a chromium-containing alloy.
In U.S. Pat. No. 4,062,737, there is described and claimed a chromium or chromium alloy electroplating solution in which the source of chromium comprises an aqueous solution of a chromium(III) thiocyanate complex and a process of plating chromium or a chromium-containing alloy comprising passing an electric plating current between an anode and a cathode in such a solution.
The preferred complexes described in said U.S. Pat. No. 4,062,737 are chromium(III) aquothiocyanate complexes prepared by equilibrating chromium perchlorate and sodium thiocyanate in an aqueous solution. The complexes so formed are described by the general formula: EQU ((H.sub.2 O).sub.6-n Cr(III).(NCS).sub.n).sup.(3-n) where n=1 to 6
(Subscripts are always positive, but superscripts may be positive, negative or zero.)
In the specification of our copending application, Ser. No. 913,639, filed June 8, 1978, entitled: "Method of and Solution for Electroplating Chromium and Chromium Alloys and Method of Making the Solution," there is described and claimed a chromium or a chromium alloy electroplating solution in which the source of chromium comprises an aqueous solution of a chromium(III) thiocyanate complex having at least one ligand other than thiocyanate or water in the inner coordination sphere of the complex. Chromium(III) species in solution are ocatahedral with six ligands coordinated to the chromium atom. These ligands occupy and define the inner coordination sphere of the chromium atom and are inert inasmuch as they exchange very slowly with free ligands in the solution, e.g., the reaction: EQU (Cr(H.sub.2 O).sub.5 (NCS)).sup.+2 +*(NCS).sup.- .fwdarw.(Cr(H.sub.2 O).sub.5 *(NCS)).sup.+2 +(NCS).sup.-
is very slow. It is the slowness of reactions of this type which complicates the chemistry of chromium(III) and necessitates equilibration at high temperatures. See the book by Basolo and Pearson, Mechanism of Inorganic Reactions: Study of Metal Complexes in Solution, published by Wiley. The linear thiocyanate anion, NCS.sup.-, has unique catalytic properties through its ability to coordinate to metal ions through its nitrogen and sulphur atoms. Also, its electron density is extensively localized across the three atoms.
The thiocyanate anion is believed to catalyze the electron transfer reaction: EQU Cr(III)+3e.sup.- .fwdarw.Cr(0)
through the formation of multiple-ligand bridges between a thiocyanate Cr(III) complex and the surface of the cathode. The electro-active intermediate can be identified as: EQU Cr(III)-NCS-M
where M is the metal surface of the cathode which is Cr(0) after an initial monolayer of chromium is plated. The "hard" nitrogen coordinates to the Cr(III) atom and the "soft" sulphur to the metal surface M of the cathode. Multiple-ligand bridging by thiocyanate in the electrochemical oxidation of chromium(II) at mercury electrodes is described in Inorganic Chemistry 9, 1024 (1970).
One embodiment of the invention described in our above-mentioned application Ser. No. 913,639 comprises a particularly advantageous chromium or a chromium alloy electroplating solution in which the source of chromium comprises an aqueous solution of a chromium(III) sulphatothiocyanate complex. More particularly, the chromium(III) sulphatothiocyanate complex comprises mixed chromium(III) thiocyanate complexes having the formula: EQU ((H.sub.2 O).sub.6-m-n Cr(III) Cl.sub.m (NCS).sub.n).sup.3-m-n
where m and n are both positive integers, but where m+n does not exceed six. Preparation of this aqueous solution of chromium(III) chlorothiocyanate complex was by equilibrating an aqueous solution of chromic chloride (CrCl.sub.3.6H.sub.2 O) and sodium or potassium thiocyanate.
Commercially, chromium has been plated from aqueous chromic acid baths prepared from chromic oxide (CrO.sub.3) and sulphuric acid. Such baths in which the chromium is in hexavalent form present a considerable health hazard as a result of the emission of chromic acid fumes. Further, if the plating current is interrupted for any reason, a deposit of unsatisfactory milky appearance is produced. In addition, delamination of the deposited chromium occurs. Thus, accidental interruption of the plating current can cause significant losses, and barrel chromium plating is rendered extremely difficult since it is difficult to apply more than very thin deposits of chromium and to ensure that the deposit covers and adheres to the articles to be plated.
Chromic acid plating baths have the further disadvantages that the plating efficiency is low and, therefore, the rate of deposition is low, the throwing power is limited, and it is difficult to deposit layers of uniform thickness over substantial areas. More metal is deposited on high current density areas, such as edges, and in certain circumstances, "burning" appears. It should also be noted that chromic acid plating baths contain a very high concentration of chromium, 100-200 grams per liter. However, since chromium salts are relatively expensive, the chromium concentration should be kept as low as possible to minimize the cost of making up the bath and to reduce "drag-out" on work pieces. The reduction in drag-out loss in making decorative chromium deposits is important since drag-out can amount to six or more times the weight of metal plated.
Numerous attempts have been made to use trivalent chromium salts to deposit chromium or a chromium-containing alloy.
The specification of United Kingdom Pat. No. 1,144,913 describes a solution for electroplating chromium, which includes chromium chloride contained in a dipolar aprotic solvent (such as dimethylformamide) and water. United Kingdom Pat. No. 1,333,714 describes another solution which includes chromium ammonium sulphate in a dipolar aprotic solvent and water.
However, such solutions possess limitations which hindered their industrial acceptance. In particular, parts of complex shapes could not be plated satisfactory and the poor electrical conductivity, due to the presence of the dipolar aprotic solvent, required a power supply capable of supplying up to 20 volts. Reduction in the quantity of the dipolar aprotic solvent resulted in an unsatable bath. In addition, the solution was relatively expensive. The plating solution also contained between 0.5 and 1.5 M chromium ions, a relatively high concentration. There are also health hazards associated with the use of dimethylformamide.
U.S. Pat. No. 3,917,517, claiming priority from United Kingdom patent application No. 47424/73, describes a chromium or chromium alloy electroplating solution comprising chromic chloride or sulphate having hypophosphite ions as a supplement to or replacement of the dipolar aprotic solvent disclosed in the last two mentioned United Kingdom patent specifications. The addition of hypophosphite ions to a trivalent chromium electroplating solution is said to "mitigate" or "overcome" many of the disadvantages of the solutions containing the dipolar aprotic solvent. However, the plating efficiency is stated to be lower than with high levels of the dipolar aprotic solvent and plating rates of 0.05 to 0.15 microns per minute, similar to the best rates available with the hexavalent chromic acid plating solutions, were obtained. Preferred range of temperature for plating is stated to be 25.degree..+-.5.degree. C. with a practical maximum being 35.degree. C. for a chromic chloride solution and 55.degree. C. for a chromic sulphate solution. The concentration of chromium was given as being 0.5 M to 1.75 M with a preferred range of 0.7 M to 1.3 M.
German Offenlegungsschrift Nos. 2,612,443 and 2,612,444, claiming priority from United Kingdom patent application Nos. 12774/75 and 12776/75, respectively, describe an aqueous electroplating solution comprising chromic sulphate having hypophosphite or glycine ions as "weak complexing agents" and chloride or fluoride ions, respectively. The maximum plating rate was again approximately 0.15 microns per minute and the preferred temperature range 25.degree.-35.degree. C. The preferred concentration of chromium for decorative plating was given as 1 M.
German Offenlegungsschrift No. 2,550,615, which corresponds to United Kingdom application No. 38320/72, also discloses a trivalent electroplating solution containing chromic sulphate or chloride, ammonium sulphate or chloride, boric acid, and a variety of alternative additional "weak complexing" materials, including glycine ions and hypophosphite ions. However, in the examples, the concentration of the additional buffer material was relatively high.
United Kingdom Pat. Nos. 1,445,580 and 1,455,841 described another approach that has been used to deposit chromium from aqueous solutions to trivalent salts. In these patents, the source of chromium ions was chromic chloride or chromic sulphate or chromic fluoride. In addition, bromide ions, ammonium ions and formate or acetate ions are stated to be essential. The plating rate was stated to be 0.15 microns per minute and a temperature in the range of 15.degree.-30.degree. C. The concentration of chromium was given as between 0.1 and 1.2 M, the preferred value being given as 0.4 M chromium ions.